The advantages of this instrument may be thus enu merated.
1. While the greater number of the sextants now made measure only to 124°. Amici's sextant can measure The former will not take double me ridian altitudes of the sun at Genoa from the 7th May till the 8th August, and under the equator•they can never be used for this purpose, but the latter will take their altitudes under the tropics and even at the zenith.
2. The points zero and 90 degrees can be verified in Amici's sextant.
3. When an artificial horizon is used, the telescope always rests in a horizontal position, so that the ob server may sit at his ease before it, whatever be the altitude of the star he is observing.
4. At sea we may take all the altitudes of two op posite horizons, both the anterior and the posterior,. the mean of which will correct the inequalities of re fraction.
5. In I Iadley's mirrors fully one half of the light is lost by reflexion, whereas in the prisms very little light is lost.
6. The mirrors are subject to flaws and cracks, and sometimes to the loss of the quicksilver, whereas no thing can injure the prisms unless a force which breaks them to pieces.
7. Amiei's sextant has no parallax for objects near the observer, owing to the great proximity of the prisms.
8. The greatest of all advantages is, that we can make the back observation with Amici's instrument without adding a third prism. Sec Baron Zach's Correspondence .Istronomique, vol. vi. p. 554.
In observing the altitudes of celestial bodies, it is necessary to see the apparent horizon, but as this is always obscured in foggy weather, and sometimes even in clear weather cannot be seen from the inter position of the coast, or an island, it became of great consequence to be possessed of what is called an arti ficial horizon. These have been constructed of ". great variety of forms, such as vessels filled with mercury, oil, or tar, pendulums, plumb lines, and levels; some of the most useful of these we shall pro ceed to describe.
The common artificial horizon consists of a shallow vessel nearly filled with mercury, or with any viscid fluid, such as treacle, tar, oil, &c. Instead of using a fluid, Troughton makes them of black glass. as in
Fig. 11. and levels them by means of the screws SSS, with a bubble laid upon the surface. Dollond con structs them as in Fig. 12. with a plate m n of clear glass, concave beneath, and filled with spirits p 9. so as to serve as a level. These two forms of the hori zon are preferable in cases where the tremor disturbs the mercury or treacle. The method of using the ar tificial horizon is shown in Fig. 13. where KL is the horizon and e f g the sextant. The image of the sun reflected at b from the mercury or tar passes through the horizon glass d to the eye at M, and is made to coincide with the image of the sun reflected from the index glass e, and seen by the eye at M. The angle thus measured is RM b, which will be twice the angle S b K or RSL, In Fig. 7. with the real horizon the angle observed is 35°, but in Fig. 13, with the artifi cial horizon, the angle is The artificial horizon will answer only for objects at a very great distance, as there will be a parallax corresponding to its dis tance MO from the eye.
In windy weather it is necessary to protect the sur face of the mercury or tar from being ruffled by means of a cover or roof, MNO, Fig. 14, consisting of two plates, MN, MO, of parallel glass inclined nearly at right angles. After one observation the roof should he reversed, and another observation made in order to correct any error arising from want of parallelism in the surface of the plates of glass.
Mr. Serson who was lost on board his majesty's ship Victory about the middle of the last century, observed that when a top was spun, its upper surface directed itself in the course of two minutes after it was set up in a true horizontal plane; that this plane was not at all disturbed by any inclination or motion of the box on which it was placed, and therefore that it might be of great advantage as an artificial horizon.* When it was spun in the open air it continued 35 minutes in motion, but when it revolved in vacuo its motion last ed two hours and sixteen minutes, preserving itself per fectly horizontal for three quarters of an hour.